Process for surface treatment of steel strip

ABSTRACT

A PROCESS FOR THE SURFACE TREATMENT OF STEEL STRIP BY PASSING A STEEL STRIP CONTAINING A CHROMIC ACID COATING THROUGH A TANK AND ELECTRODEPOSITING ON SAID STRIP AN ORGANIC FILM. THE FILM IS APPLIED WHILE THE STRIP IS MOVING AT HIGH SPEED. THE RESULTING STRIP IS CHARACTERIZED BY IMPROVED PROPERTIES, I.E., IMPROVED CORROSION RESISTANCE, EASE OF FABRICATION, ETC.

PROCESS FOR SURFACE TREATMENT OF STEEL STRIP Filed Jan. 2;)19'67 a Sheets-Sheet i Electrodepositon To Metal Surfaces .Relationship Between Electrodeposition Time And Coating Weight Type Of Coating 1 Epoxy Concentration 10 Temperature .2 25 C Voltage 2 60 V 1 v A [Chromium-Plated Steel] B [Pho hated Steel].

: 1.0- 0 3 C [Nickel-Plated Steel] E L D [Copper-Plated Steel] m 3 E[Stee\] U W; i O r U Time (Sec.)

INVENTOR. HIM MU uclmm y mossu snro nKnyn In" fl ;;1971 1 IHIIRO MU UCHIDA T L 3,558,430

PROCESS FOR SURFACE TREATMENT OF STEEL STRIP Filed Jan 2:; 1967 Z Sheet s-Sheet 3 Electrodepositio'n Velocity Type" vOf Coating Concentration 10'! Temperatuer 25'C Voltage 60V A [chr mi'qin Q Plated Steel] Electrbdepd sition: Velocity (mg-/dm'.sgc.)

' PROCE Fi l ed Jan.- 23*, 1967 H IROMU UCHIDA AL 3,558,460 $5 FOR SURFACE TREATMENT 0Fv STEEL STRIP 3 Sheets-Sheet 3 INVENTO lf/RO MU 0 MI!) IHDESHI snrfl BY 7014mm Iii/NPR United States Patent US. Cl. 204181 5 Claims ABSTRACT OF THE DISCLOSURE A process for the surface treatment of steel strip by passing a steel strip containing a chromic acid coating through a tank and electrodepositing on said strip an organic film. The film is applied while the strip is moving at high speed. The resulting strip is characterized by improved properties, i.e., improved corrosion resistance, ease of fabrication, etc.

This invention concerns a special treatment method applied on steel strip surface, in which cold rolled steel strip, after being cathodically treated in an electrolytic solution for a short period of time chiefly composed of chromic acid, can be advantageously treated by an electrodeposition coating method used in formation of thin organic coating at a high speed on its surface, to obtain steel strip characterized by improved anticorrosiveness, workability, lacquer adhesion, etc.

In this invention, the electrolytic solution having chromic acid as a major component used for the purpose of applying cathodic treatment on cold rolled strip, corresponds to one of the solutions disclosed in US. Pat. No. 3,113,- 845, Canadian Pat. No. 710,309, French Pat. No. 1,365,- 368 and the other solutions for the same purpose; namely, an electrolytic solution for electrodeposition of chromium coating film and/or chromate film on the steel strip.

Chromium-plated steel strip and chromate coated steel strip obtained from the cathodically treated strip by means of these electrolytic soltuions, are widely being used in many ways as an excellent substitute for tin plate which has hitherto been favoured as typical can-material.

However, lacquering on such chromium-plated steel strip and chromate coated steel strip are generally done, as in case of tin plate which has hitherto been used as typical can-material, mostly on cut-off sheet and partially in a coiled condition at low temperature and low speed with lacquer diluted in organic solvent; quite separate and independent coating processes from that of electrolytic treatment.

One of the reasons for the above process is that the roller coating method hitherto used was unable to provide a thin and uniform coating at a high speed and the other reason is that it was impossible to dry and cure coating film at high temperature and by direct firing because of organic solvent used. A further reason is that in the case of tin plate, the typical can-material low melting point (232 C.) of tin coating has made drying and curing the coating film at high temperature and high speed impossible; therefore, even though this coating metal is replaced by other materials, the disadvantantages associated with prior art procedures still have to be overcome.

One of the disadvantages of the prior art procedures is due to the fact that since the lacquering process is conducted at low temperature and low speed, productivity is very low; secondly such a lacquering process, cannot cope with a high speed (150-600 mm./min.) chromium plating and/or chromate treatment process and therefore, chromium plating and/or chromate treatment must be performed separately and independently from the lacquering process. In this respect, there are the following disadvantages which have to be overcome: before the finishing chromium plating and/or chromate treatment it is necessary to use intermediate steps in the process which require extra operations like drying, oiling, as well as packing, etc. to obtain anticorrosion and antiabrasion properties. It is quite clear that oiling, is harmful, not beneficial to the lacquering steps that follows.

One of the objects of this invention is that it provides a high temperature and high speed process for lacquering thin steel strip.

Another object of this invention is to provide a special surface treatment method in which a high temperature and high speed lacquering process chromium plating and/ or chromate treatment can be jointly and continuously followed by lacquering process.

Another object of this invention is to provide a surface treatment method which can product satisfactory corrosion resistance, fabrication characteristics and lacquerability to a can, even if the time of cathodic treatment in an electrolytic solution chiefly composed of chromic acid is shortened, that is, even if chromium plate and/or chromate film on the strip surface is made thinner than the film which has been currently used. Another object of this invention is to provide a special surface treatment method in which an excellent product can be obtained characterized by improved lacquer adhesion, workability and anti-corrosiveness in comparison with the ordinary product which is provided by a coating material having conventional organic solvent obtained by a roller coating process after a cold rolled strip is cathodically treated in the electrolytic solution chiefly composed of chromic acid.

To accomplish the foregoing objects, the invention lies in application of an electrodeposition lacquering step to a cold rolled strip which was treated cathodically in an electrolytic solution chiefly composed of chromic acid. The other objects, features and effects will be clarified in the following description.

The first feature of this invention is that an electrodeposition coating is applied on a strip surface on which chromium plate and/or chromate film were formed by cathodic treatment in an electrolytic solution chiefly composed of chromic acid as mentioned above.

The present invention will be described in detail referringto the attached drawings in which:

FIG. 1 indicates the relation between electrodeposition time and electrodeposited film weight in the application of electrodeposition coating on various base materials.

FIG. 2 indicates the relation between electrodeposition time and electrodeposition velocity in case of electrodeposition coating on strip material.

FIG. 3 indicates the apparatus used in employing the present process.

While the electrodeposition lacquering process has been widely used in motorcar industry, the present invention provides, for the first time, a combined electrolytic process and electrodeposition step, which originates from the fact that strip with chromium plate and/or chromate film has been found to be lacquered more efficiently as compared with steel, zinc, nickel, copper, aluminum, and phosphated materials (refer to FIGS. 1 and 2).

Coating of chromium plate and/or chromate film has much higher melting point than plated tin and in consequence, it is possible to dry and cure electrodeposited lacquer film on the chromium plate and/or chromate film at a considerably higher temperature than that on tin coating.

Electrodeposition coating has so far been adopted chief- 3 ly in the motorcar industry; electrodeposition time in this case being 1-10 minutes, moving the speed of the material to be coated in the electrodeposition tank averages 0.1- meters/min. with a dry film weight of 200-400 mg./dm. The material to be coated is generally phosphated steel or phosphated galvanized steel. The reasons why motorcar industry utilizes electrodeposition coating are due to the fact that electrodeposition coating is quite free from the danger of fire because of the use of an aqueous solution, that the coating material penetrates well into the seams of joints and that throwing power in electrodeposition is very good. However, the purposes and the objects for which this invention utilizes electrodeposition lacquering process, are quite different from the above; they are based on the new technique established through the discovery of properties and phenomena so far unknown in conventional electrodeposition coating. And thus this invention has provided an electrodeposition time of only 0.5-5 seconds not yet conceived of not only in the motor car industry but also in any other industrial field. When steel is applied with chromium plate and/ or chromate film, it is uniformly electrodeposited much faster than bare steel as shown in FIG. 2. Moreover, the property of the resulted material has been made favourable for the use of can material, as evidenced by the results shown in Table 1.

coating very easily in a short period of time and at high speed and to bake the coating quickly at high speed and temperature including direct firing. In other words, the coating material is of water base which is different from ordinary coating material which is of organic solvent base. In addition, the solid content in a wet coating film after electrodeposition is so high, i.e., it is up to about 90% and it is not necessary to expect a levelling effect of coating film in the course of drying. By making use of all these facts high temperature and high speed baking is satisfactorily done. In this case, coating weight can be 100 mg./dm. or less but this invention will be most effectively put in force when it is set under mg./dm.

As coating material for electrodeposition in this invention, epoxy-based, acrylic-based, phenol-based, alkydbased coatings, etc. are employed making use of each special property.

Furthermore, by using the process of this invention, even though the time for cathodic treatment of steel strip in electrolytic solution chiefly composed of chromic acid, is shortened, it has become clear that it is possible to manufacture a good quality product with excellent anticorrosiveness, workability and lacquerability required as for can material. In short, simple cathodic treatment of steel strip in an electrolytic solution chiefly composed of chromic acid is not satisfactory for various uses if the TABLE 1.-CORROSION RESISTANCE OF TIN PLATE, Cr-PLATED STEEL AND Cr-PLATED STEEL WITH ELEOTRODEPOSITED EPOXY COATING Immersion tests, at room temperaure:

MEK, 7 days 1% NaOH 3hours 7 days---" 1% H01 3% NaCl 7days 0 1 Epoxy coating weight=20 mg./dm. This epoxy coating can hardly be removed by acetone, although some electrodeposited coating on tin plate is easily removed by acetone.

2 Time required to observe corrosion.

As explained so far in detail, good organic film of 5-100 m.g./dm. can be obtained in a very short period of time by electrodeposition coating on cold rolled strip with chromium plate and/or chromate film and moreover, high temperature and fast baking after electrodeposition can also be effected. As a result, because of this high speed treatment, productivity has been dramatically increased in comparison with conventional roller coating process. One more result obtained is that it has become possible to make electrodeposition lacquering keep up with the precedent fast process (100-600 mm./min.) of electrolytic treatment of strip consequently obviating difficulties arising from separate operations of electrolytic treatment and lacquering process. At the same time, it has been made possible to conduct cold rolled strip after cathodic treatment in electrolytic solution having chromic acid as major component directly into electrodeposition tank from a final washing process without a drying and oiling period. Therefore, oily matter does not remain on the surface of chromium plate and/or chromate film with chemical activity, and lacquer adhesion after electro deposition is constantly good and a product is obtainet. characterized by superior anticorrosiveness, workability, etc. It has also been found that the aqueous coating solution used in this invention is free from deterioration by the chromium plate and/or chromate film.

The purpose of adopting electrodeposition as coating process in this invention is to unify the lacquering process with previous washing process, to obtain uniform Wet resulted inorganic surface film is less than 0.01 micron in thickness (chromium content: 0.5 mg./dm. However, by employing the method of this invention, applying an electrodeposition coating for 0.5-5 seconds and obtaining an organic film of 5-100 mg./dm. it is possible to obtain a product satisfactory for various uses, even if the inorganic film be less than 0.01 micron in thickness. In the method of this invention, there are many ways to be adopted as to moving strip into the electrodeposition tank. For example, it is possible to move it from the upper part of the tank downward and to take it upward by sinkroll at the bottom of the tank and then to draw it out vertically, or to move the strip upward through the bottom of the tank and then to draw it out vertically, or to move the strip upward through the bottom of the tank by making use of seal and then to draw it out of the tank. In any case, the method of this invention is intended to draw out the electrodeposited strip vertically and to dry, cure and cool the strip during the process in which it reaches so far as deflector roll installed almost directly above the electroplating tank so that the coating film cannot be scratched by deflector roll. And in order to draw out the strip vertically out of the tank, it is not necessary to have any special guide rolls during the vertical movement of the strip and the deflector roll can draw the strip out of the tank so that there is no fear of having scratches by the rolls over coating film on the strip in the course of its movement. The strip applied with chromium plate and/or chromate treatment only as well as the surface treated strip obtained through the method of this invention is used in the manufacture of food cans, crown caps, miscellaneous cans, etc. as in the case of tin plate, but the surface treated strip obtained through the method of this invention is superior to the conventionally treated products. Since prior products often oiled at the final step, there is a fear of having eye-holes in the following lacquering strip. However, the product obtained by the process of this invention avoids this problem. Further, using conventional procedures it is also diflicult to avoid scratches on the surface during the can making process, especially in case of some work done on it like drawing where it is hardly subject to scars in the course of working. Using the present process, products are obtained having superior anticorrosiveness in the air, corrosion resistance against water, salt spray and chemicals are as compared with products obtained by the use of conventional pro cedures.

As shown by the examples, the product obtained by the process of this invention is superior in properties to the product obtained by a roller coating process in a separate line from electrolytic treatment.

With respect to the apparatus illustrated in FIG. 3 applied to the process of this invention, a cold rolled steel strip uncoiled from uncoiler 1 goes through pinch roll 2, tension bridle 3, electrolytic cleaner 4, washer 5, pickling tank 6, washer 7, brushing and washer for surface cleaning 8, cathodic treatment tank 9, in which chromium plate and/ or chromate film of 0.00l0.1 micron in thickness are formed on the strip surface, washer and then the strip is conducted on to electrodeposition tank 11 in which coating material is electrodeposited, taken out vertically to deflector roll directly above, and surplus aqueous solution is pushed downward back by air knife 12. Baking is done in the drying and curing furnace 13 and at point 14 the strip is compulsively cooled by air in the course of the strips movement toward deflector roll 15. The strip finally passes through tension roll 16 to be coiled up by recoiler 17.

Some explanation based on the present examples will be supplementally given here on the method of this invention, but it should be recognized that changes in the detailed embodiments disclosed may be made without departing from the spirit of the invention or the scope of the appended claims.

EXAMPLE 1 In such continuous treatment line as shown in FIG. 3, continuous surface treatment was applied on cold rolled steel strip 0.22 mm. thick and 150 mm. wide. In this case, chromium plate (0.02 micron thick) was applied in cathodic treatment tank 9 and directly after washing at the final stage 10 of plating process, the strip in wet condition was conducted consecutively to electrodeposition tank 11 at a temperature of C. in which 8% aqueous coating solution of epoxy type is kept and organic film was electrodeposited on the anodic strip for 1.5 seconds.

After finishing the electrodeposition mentioned above, the strip was taken out upward vertically and at the same time surplus aqueous solution was pushed back downward to the electrodeposition tank. In the course of the vertical movement of strip up to deflector roll fixed directly above, the strip went through the drying and curing furnace 13 for 3.5 seconds at a temperature of 800 C. and directly after passage through this furnace, the strip was subjected to air cooling at room temperature resulting in the formation of a coating film of about 2 microns. Finally, after going through deflector roll, the strip was coiled up on recoiler 17. The line speed was about 120 mm./ min. throughout the line. No visual defect was noted on the coating film of the product.

Comparative test results obtained are shown as in the following table.

In this table, test specimens A were manufactured according to Example 1, test specimens B manufactured w1th chrom1um plating process only of Example 1, test specimens C manufactured with test specimens B coated additionally by roller coating.

In this case, a styrene alkyd lacquer was applied and baked for 10 mlnutes at 170 C., the dry film weight being 20 mg./dm.

In the following table, a full mark is five, excluding scratch test.

5-almost no rust 4l0% or less of rust on the Erichsen cup 330% or less of rust on the Erichsen cup 25 0% or less of rust on th Erichsen cup lrust all over Test pieces (a) are pushed out 5 mm. in center by the Erichsen Testing Machine, and test pieces (b) are lids of food can of 301 diameter. Scratch test results are shown with a number of rubbing time so that the abrasive defect becomes visible by rubbing test pieces with chromium coated plate which is 10 cm. long, 5 cm. wide and 200 g. in weight.

Salt spray test, 5 hours:

Test piece (a) Test piece (b) Ont door exposure test, 20 days:

Test plece (a) Test piece (a) Test piece (b) 1% citric acid test, 0., 5 hours:

Test piece (a) Test piece (b). Scratch test CD 0! O L?! ikcn N k HOJIO N10 HM NM The test results shown above indicate that test specimens A are superior to test specimens B with chromium coating only and superior to test specimens C. Specimen A is superior to specimen B especially in the outdoor exposure test and the scratch test.

EXAMPLE 2 EXAMPLE 3 In Example 1, a cathodic treatment tank 9 at a temperature of 30 C. was filled with a electroytic solution lncluding chromic acid 35 g./l. and sulphuric acid 0.4 g./l., and the strip was cathodically treated for 4 seconds at the current density of 15 a./dm. And then the strip was electrocoated for 3 seconds in the electrodeposition tank filled with 5% aqueous solution of the alkyd type and passed through the drying and curing furnace for 6 seconds at 600 C. with a line speed of m./min. This strip was characterized by the same results seen in Example 1.

What is claimed:

1. In a continuous rapid process for the electrolytic surface treatment of steel strip, wherein the steel strip is first cathodically treated in an electrolytic solution composed chiefly of chromic acid, by passing the strip through the solution, whereby a chromium and/or chromated steel strip is obtained, the improvement which comprises that the plated strip is immediately thereafter passed through an electrodeposition tank containing an aqueous electrodeposition solution to electrodeposit an organic film on said plated strip, said strip acting as anode and that thereafter the strip is vertically drawn out of said electrodeposition tank toward a deflector roll installed essentially directly thereover while drying, baking, and cooling the strip in the course of its vertical movement from said tank to said deflector roll, whereby a thin organic film is formed on said chromium plated and/or chromated steel strip.

2. A process for surface treatment of steel strip according to claim 1, wherein the chromim plate and/or chromate film which are formed on steel strip by the cathodic treatment in electrolytic solution chiefly composed of chromic acid are 0.001 to 0.1 micron thick or 0.05 to 5 mg./dm. in terms of chromium content.

3. A process for surface treatment of steel strip according to claim 1 wherein the electrodepositing is done onto the strip consecutively and continuously after final washing in the cathodic treatment of the strip in electrolytic solution chiefly composed of chromic acid.

4. A process for surface treatment of steel strip according to claim 1, wherein the organic film electrodeposited is 5 to 100 mg./dm.

5. A process for surface treatment of steel strip according to claim 1 wherein the organic film is electrodeposited on steel strip which is moving at a speed ranging from 100 to 600 m./min. in the electrodeposition tank.

References Cited UNITED STATES PATENTS 1,589,329 6/1926 Sheppard et al 204183 1,641,322 9/1927 Czapek et al. 204-181 2,214,876 9/1940 Clark 204-181 2,337,972 12/1943 Clayton et al. 204-181 2,386,634 10/1945 Robinson 204181 2,640,024 5/1953 Palmateer 204181 2,707,703 5/1955 Dorst 204181 2,898,279 8/1959 Metcalfe et al. 204l81 2,956,937 10/1960 Thomson 204181 3,093,511 6/1963 Weisel et a]. 204-181 3,113,845 12/1963 Uchida et al. 204-41 JOHN H. MACK, Primary Examiner N. A. KAPLAN, Assistant Examiner US. Cl. X.R. 204-41, 183 

